A circuit arrangement of this kind is generally known and is often used, for example, in motor vehicles to check the switching in and switching out of a load such as the brake light of the motor vehicle. With the aid of the circuit arrangement, the situation is to be achieved that a fault in the switch, which switches the load in and out, and/or a fault in the load itself is automatically detected.
If, for example, the load is connected to the supply voltage and the switch is connected to ground, then a low potential must be present at the connecting circuit node in a fault-free case wherein the switch is closed. If, in contrast, a high potential is present at the connecting circuit node, this means that a short circuit of the connecting circuit node to the supply voltage is present; that is, the load, for example, has a fault and defines a short circuit. This first state can be clearly detected by the first circuit.
If the switch is open, a high potential must be present at the connecting circuit node in the fault-free case. However, if, for example, the switch is defective and forms a short circuit to ground, or if the connecting cable of the load to the supply voltage is disconnected so that an interruption is present, these two states can be detected in that a low voltage is present at the connecting node when the switch is open. However, the two states cannot be clearly distinguished by the first circuit.